An essential component in many microwave and RF circuits is the balanced to unbalanced transformer or balun. Balun applications include balanced mixers, multipliers, and amplifiers for cancellation of even order intermodulation products, power amplifiers for push-pull power combining, and for the connection of naturally balanced antenna structures to unbalanced microwave components.
Low frequency baluns leverage ferrite and air coil transformer technology to achieve high performance and very broad bandwidth. However, at microwave frequencies it becomes increasingly difficult to fabricate this class of balun and other techniques become necessary. Miniaturization of the balun to a size and planar form compatible with monolithic microwave integrated circuit, MMIC, fabrication further complicates the design and implementation of the balun.
Due to the unavailability of ferrite technology to provide for ultrawide bandwidth baluns, there is a need to provide other types of technology to achieve a 10:1 frequency bandwidth such as between 2 Gigahertz and 20 Gigahertz. Uses for such wide bandwidth baluns include microwave surveillance applications where wideband frequency coverage is required to be able to detect the large variety of signals that pop up, as well as to provide interface circuitry for wideband digital signals and wideband antenna structures.
In general, in the basic operation of a balun, balanced signals that come out of the balun from an unbalanced line input are equal in amplitude and 180 degrees opposite in phase. Thus, for instance, signals that come in on an unbalanced 50-Ohm line may be converted to signals applied to a balanced 300-Ohm line.
One of the main purposes of baluns other than for impedance matching is for the cancellation of second-order distortion of which second harmonics is a part. As such, such baluns can be used for balanced mixers so that when a signal comes into the single-ended input of the mixer, it comes out split into a different kind of signal.
The balun can be used with differential amplifiers or pairs of amplifiers that connect to an antenna, with the balun being used for combining the output of the differential amplifier so that it does not lose half of its power into a termination.
While it is known to cancel second-order distortion in the front ends of amplifiers and to do so in the microwave region of the electromagnetic spectrum, the problem is that in an amplifier, to cancel second harmonics the balun has to be operating from the lowest input frequency to twice the highest output frequency. Thus, if one has an amplifier that operates for instance between 1 and 5 Gigahertz, the balun would have to operate between 1 to 10 Gigahertz to cancel the second harmonics.
Note that harmonics and other non-linearities in an amplifier create distortion in which harmonics constitute one form of distortion. Other forms of distortion can be intermodulation products, and the mixing of two signals that creates spurious tones at the sum and difference frequencies. These are all second-order products that need to be canceled. The cancellation of spurious tones using second harmonic cancellation is the operating province of balanced mixers that help to cancel these tones.
Moreover, it is noted that in a receiver, a receiver would have to be able to deal with every single spurious tone that would appear at the output of its pre-amplifier. It is noted that in an amplifier of bandwidth greater than an octave, the highest-power distortion tones are usually the second order tones
The effect of such distortion can be seen as follows. If one is operating near a radio station and one happens to be listening to a weaker station further away, the presence of the large signal from the nearby radio station can create spurious tones due to the above-mentioned non-linearities in an amplifier. This makes it virtually impossible or very difficult to receive the signal that one is interested in with good fidelity.
Priorly, in order to reduce the second-order distortion, a brute-force approach has been employed. By increasing the power handling capability of the amplifier, it becomes more linear for a given signal level. Thus, second-order distortions were kept to a minimum by increasing the power handling capability of the amplifier and operating it at reduced input signal levels.
However, this is not an acceptable approach, especially in cases where one wants to use battery-powered devices or if one wishes to have a number of these amplifiers located in a small space, such as in a phased array.
While the technique of using a balun for second-order distortion cancellation is a known technique, there has been no ability to apply the balun to the kinds of frequency bandwidths that are required in signal intelligence applications because baluns do not typically have a 10:1 bandwidth ratio in the microwave region of the spectrum.
While it is possible in the HF region of the electromagnetic spectrum to broaden the bandwidth of the balun through the use of ferrites, normal ferrites do not work at microwave frequencies. While in the past microwave ferrites have been developed, they do not operate with a low enough loss to achieve the required bandwidths.